The invention relates to an improvement in a sewing machine having a table, driven in two directions by two motors and accommodating material to be sewn, sewing tools and a cutting device for producing a buttonhole in the material to be sewn. The buttonhole is defined by a buttonhole bead which is formed of zigzag stitches, running around an incision, the incision being produced either before or after the sewing of the buttonhole bead. The sewing tools comprise a needle bar, which is driven up and down and oscillates in the horizontal direction, and a needle, which is provided at the bottom end of the needle bar and interacts with a looper mounted in the base plate, the needle being drivable in a rotatable manner by a third motor. The sewing machine has a control device, from which various buttonhole shapes stored therein can be retrieved.
DE 33 02 385 A1 discloses a conventional sewing machine of the foregoing type. Stepper motors are used, so that this sewing machine can be controlled digitally. The table motion can be controlled by a suitable control program in such a way that various buttonhole shapes can be cut and sewn or stitched. Complicated cam mechanisms are not necessary for producing the buttonhole bead, in either the pre- or the post-cutting mode.
In the pre-cutting mode, the material to be sewn is cut first and then the buttonhole bead is produced around the incision; in the post-cutting mode, first the buttonhole bead is produced and then the material to be sewn is cut. In the pre-cutting mode, it is necessary for the opposite seam rows to lie exactly next to one another, so that fraying of the pre-cut material is prevented. On the other hand, in the post-cutting mode, an intermediate space (intermediate material within the buttonhole bead) must remain clear between the two opposite stitch rows forming the buttonhole bead, so that the subsequent cutting of the buttonhole cuts only the material to be sewn and not the already-sewn buttonhole bead.
In the known sewing machine, it is not apparent how the bead can be precisely positioned in the pre- and post-cutting modes as described above, or how the sewing machine can switch from the pre-cutting mode into the post-cutting mode. It is also not apparent how the zigzag stitches are produced.
There are several known methods of producing zigzag stitches. According to one method, the table, to which the material to be sewn is clipped, may be driven in an oscillating manner transversely to the sewing direction, so that the successive points pierced by the needle are offset in the transverse direction.
U.S. Pat. No. 1,991,627 discloses an eyelet-buttonhole sewing machine in which the needle bar is deflected in the horizontal direction by an oscillating shaft in order to produce the zigzag stitches, the oscillating shaft being coupled to the arm shaft via a drive mechanism.
DE 34 01 615 C2 discloses a standard sewing machine in which a zigzag motion of the needle bar is initiated via a stepping motor. However, buttonholes cannot be produced with this sewing machine.
DE 41 32 586 C2 discloses an eyelet-buttonhole sewing machine in which the intermediate material which must be provided in the post-cutting mode is defined in the buttonhole bead by displacing the sewing pattern for producing a so-called offset. This offset is imparted to the transport table, which is driven by two stepper motors arranged along respective axes (x, y) disposed perpendicularly to one another. That is to say, before the forming of the stitches is started, the transport table, on which the material to be sewn is located, is put into such a position that the inner stitch, provided opposite the subsequent incision point, of the needle maintains a corresponding distance from the opposite stitch. By the storage of different data records via which the stepping motors are activated, the offset can be activated according to operating mode.
A disadvantage with this machine is that the coordinate values of the individual piercing points, that is, the activating data for the x- and y-motors, have to be re-calculated as a function of the desired offset value and the desired dimension of the intermediate material, and filed in suitable memories. If steps are lost when approaching the initial position, there is the risk of the stitches being staggered in the pre-cutting mode and of the already-formed buttonhole bead subsequently being cut open as a result.
It is also a disadvantage with this machine that two motors have to be controlled individually. The risk of data loss or of the occurrence of computing errors exists individually for each motor, so that the risk is doubled.
The disclosures of all prior art mentioned herein are expressly incorporated by reference.
In view of these problems, it would be desirable to improve the sewing machine of the conventional type in such a way that it works reliably in the pre- and post-cutting modes, and, if the sewing pattern is changed, the data for the activation of the drive motors for the table can be kept unchanged.